Controllers to start and stop irrigation cycles without human intervention are well known. These controllers send an electric current (usually 24 volt alternating current in horticultural or agricultural use) to a remote solenoid valve, causing the valve to open. Valve closure is usually effected by discontinuing the supply of electric current to the solenoid of the valve whereupon the valve is caused to close.
Most of these types of controllers are able to handle a number of valves, opening and closing them in a programmed succession for programmed times on programmed days of the week. This series of sequential valve opening and closing on specified days is generally referred to as “a program” or “an irrigation program”. Many of the known controllers are capable of storing and executing more than one irrigation program, which adds a degree of flexibility to what the controller may accomplish.
Basically these prior controllers fall into one of three categories as follows:
1. Relatively inexpensive controllers which are capable of executing an irrigation program. These controllers are not capable of changing the set irrigation program in any way to take account of differing water needs of plants occasioned by variations in meteorological conditions.
Controllers of his type constitute well over 90 percent of all irrigation controllers currently in use in Australia. Such controllers will, if the irrigation program is not regularly modified inevitably waste considerable quantities of water, since it will be programmed to supply sufficient water to serve the needs of the plant being irrigated during periods when plant demand for water is high. Thus when the same application of irrigation water continues during periods of low plant water requirement. wastage occurs.
The potential to save water by in effect harvesting rainfall by discontinuing irrigations until that rainfall finds its way into the root-zone and is transpired by the plants, is lost unless the controller can be manually de-activated. When managing large numbers of such controllers, particularly over a wide area, it is generally not possible to manually de-activate them and re-activate them when irrigation should commence.
Additionally, such controllers are incapable of responding to occurrence of rain periods unless coupled to some specialist sensor designed for the purpose. Whilst such sensors are known they tend to be either expensive (and consequently little used) or unreliable (and again little used).
2. More expensive controllers which can alter the frequency and amount of irrigation, either up or down, as time passes in an effort to match applications to plant requirements. Such devices usually impute likely plant requirements by use of meteorological averages developed from examination of many years of meteorological records relating to the geographical area under consideration. This type of controller is an improvement upon the first described type of controller, but is still arbitrary and inflexible as it relies on averages that must inevitably waste water when the predicted conditions do not occur. Additionally, there can be no improvement in harvesting rainfall.
3. Expensive controllers which either accept direct input from automatic weather stations, or accept meteorological information directly or indirectly from a remote weather station or climatic recording facility. These controllers use such information to modify a basic program so that irrigation water applications are substantially in accord with actual plant requirements. These controllers may also be activated to apply a predetermined irrigation cycle when instructed to do so by a remote software program which accepts meteorological input and maintains a water budget for the area. However, such controllers do not utilise localised rainfall measurement and consequently irrigation management depends upon rainfall information indicative of a wider area than the irrigation area. Water wastage can result. Further, these controllers must be part of a very wide network which means that over a wide area very considerable telephony or radio costs are necessarily involved.
Another approach is described in our pending patent application no. PCT/AU97100056 the content of which is incorporated herein. In that application, the control system is based upon a method of irrigating land which includes the steps of:                (a) measuring one or more weather conditions in a first area;        (b) measuring rainfall in a sub-area of the first area;        (c) monitoring the measurements;        (d) calculating a moisture content value for the sub-area from the measurements and a predetermined moisture loss for the sub-area; and        (e) regulating the irrigation of the sub-area.        
Key to this approach is the combined use of one or more weather conditions in the first area and the rainfall in the sub area. In implementing certain forms of that invention, it has become apparent that where large numbers of sub-areas (such as parks, gardens and sports facilities) need to be managed by the system, special practical economic difficulties may arise.
If individual actual measurement is needed of a large number of sub-areas, it would be necessary to place at least one weather station including a tipping bucket (or other) type of pluviometer, in an appropriate position in each sub-area. This could be as often as 500 meters apart. However, weather stations are expensive and a very common characteristic of rainfall is that it can be extremely variable in amount and distribution even over a small area. For example, well over 1000 stations would be needed in even a small city to establish a network. Thus to produce reliable data using such pulviometers is an expensive undertaking.
Accordingly, improved and more economic resolution of rainfall information over a wide area is important in the management of individual sub-areas. In addressing this issue it has been found that one source of potentially useful data for this purpose is readily available.
In this respect, at present meteorological weather radars are commonly installed to cover the area of major cities. The output of these radars is a stream of data organised in the following fashion:                Radially—each degree of rotation from 000 through to 360 is reported separately.        Longitudinally—for each degree of rotation, data is presented as a series of rain intensity figures, typically for each kilometre along each of the radii. For example 095/35/9 may mean that rain intensity of “9” is falling 35 kilometres from the radar transmitter on a bearing of 095 degrees from the transmitter.        
This data is analysed by a high-speed computer to produce the familiar radar screen views commonly seen on television weather reports.